A self-contained, fully-integrated, energy efficient heating and cooling system or apparatus for conditioning the air on the interior of a building which includes a means for creating airflow through the apparatus, a section for cooling the airflow, a section for heating the airflow, a means for directing the airflow into the cooling section or into the heating section, a reheat device for tempering (i.e., reheating) the airflow exiting the cooling section, a return air section for receiving air returned to the apparatus from the building interior, and a means for pulling the returned air out of the return air section and venting the returned air to the environment outside of the building. The heating section of the present invention also provides a "four-pass" heat exchanger which may be assembled from sections of cast iron or other suitable materials, and a high efficiency combustion burner which may be integrated into the four-pass heat exchanger.